Author here. Right now it's being tested by running precompiled binaries. See https://github.com/jart/blink/tree/master/third_party/cosmo Most of these test binaries are sourced from the cosmopolitan repository, and many of those come from other projects like nsync, mbedtls, etc. I've recently started working on a unit testing method that's written in plain assembly, to hopefully catch more of the things that would show up in opcodes that compilers don't generate. See https://github.com/jart/blink/tree/master/test/asm